1. Field of the Invention
This invention relates to a polyethylene valve assembly and, more particularly, to a valve assembly having a polyethylene valve body formed by electrofusing a coupling to pipe end fittings.
2. Description of the Prior Art
In the underground conveyance of natural gas polyethylene conduit is replacing iron and steel conduits which were predominantly used prior to the development of polyethylene conduit for distribution service lines. The metal composition of iron and steel conduit used underground makes it susceptible to corrosion and deterioration due to cathodic action.
Not only is polyethylene being used to fabricate gas pipelines, it is also being used in the fabrication of valves for controlling gas flow through the pipelines. Polyethylene ball valves and plug valves are commercially available for installation in gas distribution systems. U.S. Pat. Nos. 3,656,711; 4,348,006; 4,523,740; 4,553,562; and 4,705,058 are representative of known polyethylene control valves. The ends of a polyethylene valve are conventionally connected to polyethylene conduit by butt fusion or by an electrofusion coupling.
Polyethylene conduit used for natural gas service provides the long term benefits of corrosion resistance and durability. The joints between pipelines and polyethylene valves can be made without the need for conventional mechanical connections. The use of butt fusion and electrofusion to connect the end fittings of a valve with a pipeline eliminate the need for conventional threaded connections and fastening nuts.
In a conventional electrofusion system a sleeve of thermoplastic material, such as polyethylene, includes a coil of resistance heating wire encapsulated within the body of the sleeve. The heating wire extends the length of the sleeve. The ends of the heating wire are connected to terminals for supplying electrical current from power control equipment to the wire. The ends of a pair of polyethylene (PE) pipes are pressed into the sleeve.
Electrical connections to the sleeve terminals are made, and then a predetermined current is supplied for a controlled period of time to heat the sleeve. The sleeve is heated to its melting temperature so that the thermoplastic material forming the sleeve softens. Heat from the sleeve is transmitted by conduction to the pipe ends at the interface with the sleeve. The thermoplastic material of the pipe ends softens so that the material forming the sleeve and the pipe ends flows together.
When the sleeve and the pipe ends have been heated to a melting temperature and the pipe ends are fused to the sleeve, current to the coupling is terminated and the sleeve is allowed to cool. Once the sleeve cools to a preselected temperature, the pipe ends and the sleeve are fused together to complete the electrofusion coupling. The joint formed by the electrofusion coupling is stronger than the pipe. In other words, in a shear test the pipe fails before the joint between the pipe ends and sleeve fails. U.S. Pat. Nos. 4,486,650; 4,571,488; 4,602,148; 5,229,581; 5,280,670; and 5,363,541 disclose electrofusion systems for forming joints between PE pipes.
Valves fabricated of thermoplastic material are composed of parts which are injection molded. Because of the size and expense of injection molding equipment, it has been difficult to economically fabricate PE service valves, such as ball valves, having end connections greater than four inches in diameter. This substantially limits the use of thermoplastic valves in natural gas distribution systems where it is preferred to connect valves to pipelines of a diameter of four inches and larger.
It is also known to use metal valves in a PE pipeline. One of the problems encountered in combining metal valves and PE conduit is coupling the members to form a fluid tight seal. The metal components must be sealingly engaged to the PE pipe. This generally requires some type of mechanical connection to be completed between the valve metal components and the PE pipe.
It is well known to use compression fittings to connect metal valves to PE conduit. Examples of transition fittings to join metal valves and PE conduits are disclosed in U.S. Pat. Nos. 2,933,428; 3,025,086; 4,005,880; 4,682,797; 4,712,809; 4,997,214; and 5,265,652.
There is need for a plastic valve in gas distribution and service systems that can be fabricated at a cost substantially less than that associated with injection molding of PE valves and provide the high performance capabilities achieved with metal valves. The plastic valve must be economically manufactured for a wide range of sizes, for example, from 1/2 inch to eight inches in size. The PE valve must be constructed in a manner that allows modification to be made so that the valve can be custom designed to meet installation requirements at a commercially competitive cost.